In media handling assemblies, particularly in printing systems, accurate and reliable registration of the sheet as it is transferred in a process direction is desirable. In particular, accurate registration of the sheet, such as a sheet of paper, as it is delivered at a target time to an image or ink transfer zone will improve the overall printing process. There are at least three degrees of freedom in which the sheet can move, which need to be controlled in order to achieve accurate delivery thereof. Accurate sheet registration refers to the control or correction of those degrees of freedom in order to deliver a sheet as precisely desired. The sheet is generally conveyed within the system in a process direction. However, often the sheet can shift in a cross-process direction that is lateral (side ways) relative to the process direction. Also, the sheet can even acquire an angular orientation, referred herein as “skew,” such that its opposed lateral edges are no longer parallel to the process direction. Further, the sheet velocity may need adjusting in order to timely arrive at a delivery or transfer point (a datum) with a desired speed.
A slight skew, lateral misalignment or arrival time or velocity error of the sheet through a critical processing phase can lead to errors, such as image and/or color registration errors relating to such arrival at a printing station. Contemporary systems transport a sheet and deliver it at a target time to a “datum,” based on measurements from one or more of the sheet's edges. The datum can be a particular point in a transfer zone, a hand-off point to a downstream nip assembly or any other target location within the media handling assembly. The lead edge, trail edge, and side edge measurements generally determine whether registration errors exist. After such registration errors are identified, the errors may be corrected prior to delivery at the datum. As the sheet is transferred between sections of the media handling assembly, without registration measurements and corrections the amount of registration error can increase or accumulate, causing pushing, pulling, or shearing forces to be generated, which can wrinkle, buckle, or even tear the sheet(s).
FIG. 1 is a schematic example of a contemporary sheet registration system. The sheet registration system consists of two sets of drive nips, an inboard nip 20 and an outboard nip 30. The nips 20, 30 are mounted with bearings on a shaft 25 so that they are free to rotate. An angular velocity is imparted to each driven nip wheel with a motor, generally controlled by a processing device, referred to as the controller (not shown). Also, the nips 20, 30 are often mounted such that they can be moved laterally along a y-axis. In fact, the motors, nips and related assemblies can all be mounted on a carriage that can move along the y-axis in order to collectively correct lateral registration errors of a sheet engaged by the nips 20, 30, as is further disclosed in U.S. Pat. No. 5,094,442 to Kamprath et al., issued Mar. 10, 1992, and U.S. Pat. No. 6,533,268 B2 to Williams et al., issued Mar. 18, 2003 and U.S. Pat. No. 6,575,458 B2 to Williams et al. issued Jun. 10, 2003 disclose alternative mechanisms for adjusting a sheet's lateral position with an appropriate actuator. These contemporary methods more generally disclose that the nip assemblies can be used to move the sheet in three degrees of freedom, i.e. process, lateral and skew, in order to achieve proper sheet registration.
The skew orientation and time of arrival of a sheet leading edge LE into a sheet registration system is typically measured by two laterally spaced leading edge sensors Si, So located just downstream of and immediately adjacent to the registration nips 20, 30. A sheet velocity actuator commanded by the controller then executes a command profile in order to timely deliver the sheet to the datum with a prescribed sheet velocity. The sheet velocity can be temporarily sped up or slowed down in order to arrive at the datum at the correct time, with a further change to a target velocity just before delivery. Also, the controller can prescribe different velocities Vi, Vo for the nips 20, 30 to generate, in order to deliver the sheet to the registration datum D with a particular skew or lack thereof. By adjusting the difference between the inboard and outboard sheet velocities Vi, Vo, a skew velocity rotates the sheet as desired, which is used to achieve sheet skew registration. In this way, laterally spaced apart differentially driven drive rollers that are part of the nips 20, 30 are used to adjust a sheet delivery time, velocity and orientation. Examples of sheet registration systems with laterally spaced apart differentially driven drive rollers include the U.S. Pat. No. 4,971,304 to Lofthus, issued Nov. 20, 1990; U.S. Pat. No. 5,169,140 to Wenthe, Jr., issued Dec. 8, 1992; U.S. Pat. No. 5,219,159 to Malachowski et al., issued Jun. 15, 1993; U.S. Pat. No. 5,278,624 to Kamprath et al., issued Jan. 11, 1994; U.S. Pat. No. 5,794,176 to Milillo, issued Aug. 11, 1998; U.S. Pat. No. 6,137,989 to Quesnel, issued Oct. 24, 2000; U.S. Pat. No. 6,168,153 B1 to Richards et al., issued Jan. 2, 2001; U.S. Pat. No. 6,533,268 B2 to Williams et al., issued Mar. 18, 2003; and U.S. Pat. No. 6,866,260 to Williams et al., issued Mar. 15, 2005, the contents of which are incorporated herein by reference.
In order to print onto both sides in a duplex printing environment, the sheet gets inverted, thus making the previously trailing edge TE the new leading edge. Unfortunately, sheet cut errors often cause the leading and trailing edges to be non-parallel. This introduces improper skew error measurements if the leading edge sensors Si, So are used for skew measurements of the second side. Thus, contemporary systems provide side edge sensors E1, E2 in order to measure skew using a common side edge. While such a system only needs one leading edge sensor, with all the edge measurements (leading edge and side edge) being taken when the sheet first enters the nips 20, 30, the system is limited to open loop control. In other words, continuous or repeated monitoring and/or adjustment of sheet registration between the leading edge sensors Si, So and the delivery datum D (referred to as closed loop control) can not be performed. Alternative contemporary systems place the second side edge sensor E2 downstream of the nips 20, 30, but well before the datum D, which enables closed loop control. However, positioning the second side edge sensor E2 downstream of the nips 20, 30 has the disadvantage that skew control can not start until the edge of the sheet reaches the downstream sensor E2. This delays or does not leave very much time or distance to make corrections before the sheet reaches the datum D. While two leading edge sensors Si, So can once again be used to measure initial skew, the above noted errors in a duplex environment limit the effectiveness of such a system. Also, this increases the number of required sensors, which increases production and maintenance costs.
Accordingly, it would be desirable to provide a method, system, and printmaking device for accurately registering a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.